Takaaki Tsurumi

Enhanced piezoelectric properties of barium titanate single crystals with different engineered-domain sizes

For tetragonal barium titanate (BaTiO3) single crystals, an electric field (E-field) applied along the [111]c direction can induce an engineered-domain configuration in these crystals. In this study, such engineered-domain configurations of different domain sizes were induced in BaTiO3 single crystals, and their piezoelectric properties were investigated as a function of domain size. Prior to this study, the dependences of the domain configuration on the temperature and E-field were investigated using a polarizing microscope in order to understand the optimum poling condition for fine- and coarse-domain configurations. We found that above the Curie temperature (TC) of 132.2 °C, when an E-field above 6.0kV∕cm was applied along the [111]c direction, an engineered domain with a fine-domain configuration appeared. Moreover, it was also found that this fine-domain configuration remained stable at room temperature without the E-field. On the other hand, the coarse-domain configuration was obtained upon poling a...

Preparation of Lead Zirconate Titanate Thin Film by Hydrothermal Method

A hydrothermal method for preparing thin films of crystalline PZT was developed by controlling the rates of nucleation and crystal growth. This method consisted of two steps of hydrothermal reaction. The first step (nucleation process) was that in which the TiO2 substrate reacted with the mixed solution of Pb and Zr to form PZT and/or PZ nuclei on the surface. Subsequently, the crystal growth of PZT was promoted as the next hydrothermal step (crystal growth process) by the reaction of the mixed solution of Pb, Zr and Ti. From the experimental results of PZT powder preparation, the conditions of the nucleation and crystal growth process were determined as at 150°C for 24 h and at 120°C for 48 h, respectively. It was verified that the thin films consisted of PZT polycrystals and showed ferroelectric properties.

Preparation of nm-Sized Barium Titanate Fine Particles and Their Powder Dielectric Properties

Barium titanate (BaTiO3) crystallites with various particle sizes from 17 to 100 nm were prepared by the 2-step thermal decomposition method of barium titanyl oxalate (BaTiO(C2O4)24H2O). The crystal structure of these BaTiO3 particles was assigned to cubic m-3m by a X-ray diffraction (XRD) measurement while it was assigned to tetragonal 4mm by a Raman scattering measurement. Investigation of impurity in these particles using both TG-DTA and FT-IR measurements revealed that no impurity was detected in the BaTiO3 lattice while hydroxyl and carbonate groups were detected only on the surface. The dielectric constants of these powders were measured using suspensions by a modified powder dielectric measurement method. As a result, the dielectric constant of BaTiO3 particles with a size of around 70 nm exhibited a maximum of over 15,000. This study revealed that BaTiO3 particles with a size around 70 nm were the most desirable for capacitor application.

Fabrication of Barium Titanate/Strontium Titanate Artificial Superlattice by Atomic Layer Epitaxy

Epitaxial growth of BaTiO3, SrTiO3 and their superlattices was carried out by the alternating evaporation method using an atomic-layer-epitaxy control system for molecular beam epitaxy (MBE). Stable and reproducible control of the chemical composition was achieved by layer-by-layer growth with this system. Both BaTiO3 and SrTiO3 were grown epitaxially on SrTiO3 substrates, and clear oscillations of reflection high-energy electron diffraction (RHEED) intensity were observed, corresponding to the alternating depositions of BaO and TiO2 in BaTiO3 growth. The artificial superlattice with the structure of [(BaTiO3)5/(SrTiO3)5]7 was fabricated, and the X-ray diffraction (XRD) pattern of the superlattice was simulated by a step model. Diffraction angles of satellite peaks and the shape of Laue peaks in XRD patterns were consistent with those calculated, indicating that the designed structure was fabricated in the superlattice.

Composite structure and size effect of barium titanate nanoparticles

Almost impurity and defect-free barium titanate (BaTiO3) nanoparticles with various sizes from 20 to 430 nm were prepared using 2-step thermal decomposition method. The nano-structures of these particles were analyzed using a synchrotron radiation X-ray diffraction (XRD). As a result, it was found that the BaTiO3 nanoparticles had composite structure consisted of (a) internal tetragonal layer, (b) Gradient-Lattice-Strain Layer (GLSL) and (c) surface cubic layer.

Size effect on the crystal structure of barium titanate nanoparticles

A size effect on crystal structure has been investigated for barium titanate (BaTiO3) nanoparticles of 40-, 140-, and 430-nm sizes, by means of neutron and high-resolution synchrotron x-ray powder-diffraction and Raman-scattering techniques. These samples were prepared by a modified two-step thermal decomposition method from barium titanyl oxalate, resulting in very few lattice impurities. Rietveld analysis of the neutron-diffraction data for the 430-nm- and 140-nm-sized BaTiO3 particles was performed assuming a single phase of tetragonal (P4mm) structure. The axial ratio c∕a of tetragonal BaTiO3 decreases with a decrease in particle size from 430 to 140 nm. Barium titanate particles with a size of 40 nm consist of (1) tetragonal crystals (83 wt %) with a large cell volume and an axial ratio of unity c∕a=1.000(5) and of (2) a hexagonal phase (P63mmc, 17 wt %) with a large unit-cell volume. Rietveld and maximum-entropy method analyses suggest that there exist atomic displacements from the ideal site of a c...

Size and temperature induced phase transition behaviors of barium titanate nanoparticles

High density, almost impurity-free and defect-free barium titanate (BaTiO3) fine particles with various sizes from 20to1000nm were prepared by the two-step thermal decomposition of barium titanyl oxalate and postheating treatment. The crystal structures of these particles were investigated as a function of the size and the temperature using synchrotron radiation x-ray diffraction (XRD) measurement. As a result, the size-induced ferroelectric (tetragonal-cubic) phase transition observed at around 30nm. Moreover, the temperature dependence of the crystal structures revealed that one phase transition at 135°C separated into two kinds of phase transition behaviors with decreased particle sizes, i.e., the tetragonal-cubic phase transition temperature was constant at 135°C despite particle sizes while the cell volume expansion temperature shifted to low temperature with decreasing particle sizes. Moreover, the temperature dependence of Raman scattering spectra clarified that the temperature at a discontinuous c...

Domain Size Effect on Dielectric Properties of Barium Titanate Ceramics

Barium titanate (BaTiO3) ceramics with various grain sizes were prepared by a conventional sintering method and a two-step sintering method. The permittivity of the ceramics increased with decreasing the grain size down to 1.1 µm on average. The BaTiO3 ceramics with an average grain size of 1.1 µm had a high permittivity of 7,700. The transmission electron microscopy (TEM) observation revealed that the 90° domain density increased with decreasing the grain size. The domain size of the ceramics with the highest permittivity of 7,700 was approximately 100 nm. From an ultra wide range dielectric spectroscopy, it was found that the high domain density enhanced the orientational polarizability due to the domain-wall vibration and the ionic polarizability due to the lattice vibration. It was clarified that the increase of the permittivity with decreasing the grain size was due to the domain size effect.

Poling Treatment and Piezoelectric Properties of Potassium Niobate Ferroelectric Single Crystals

Piezoelectric properties of potassium niobate (KNbO3) single crystals were investigated as a function of crystallographic orientations, i.e., [110]c of polar direction and [001]c of nonpolar direction. Prior to the piezoelectric measurements, the optimum conditions for a conventional poling method were investigated, and it was found that the optimum conditions for a conventional poling method were as follows, i.e., (a) high temperature above 100°C, (b) long soaking time over 1 h, (c) slow heating and cooling rates below 1°C/min, and (d) lower poling current below 1 µA. However, fully poled KNbO3 crystals were not obtained using the conventional poling method. Thus, to achieve fully poled KNbO3 single crystals, a new 2-step poling method was proposed. This method is composed of 2 poling stages, i.e., the 1st step for non-180° domain switchings at higher temperatures under a low DC bias field, and the 2nd step for 180° domain switchings at lower temperatures under a high DC bias field. Using the 2-step poling method, KNbO3 crystals were successfully poled, and then, their piezoelectric properties of k31 modes were measured using a resonance method. The [110]c poled KNbO3 crystals exhibited electromechanical coupling factor k31 of 28.9% and piezoelectric constant d31 of 18.4 pC/N while the [001]c poled KNbO3 crystals exhibited k31 of 31.2% and d31 of 51.7 pC/N. The piezoelectric constant d31 along nonpolar [001]c direction was 2.8 times higher than that along polar [110]c direction.

Growth condition dependence of morphology and electric properties of ZnO films on sapphire substrates prepared by molecular beam epitaxy

Zinc oxide (ZnO) films were grown on sapphire (1120) substrates by molecular beam epitaxy under oxygen radical irradiation. The effect of the growth conditions, including the Zn/O ratio supplied to the film surface, on the electrical properties of ZnO films was studied in relation to the film morphology. We found that the growth rate strongly depended on the Zn flux from the Knudsen cell and the optimum condition for high growth rate was very narrow. The grain size in the lateral direction increased with increasing growth rate in the thickness direction. The increase in growth rate, especially in the lateral direction, resulted in the carrier mobility increasing up to 42 cm2 V−1 s−1. The carrier concentration N was sensitive to the substrate temperature, while the value of N was not sensitive to the source supplying ratio Zn/O. We discuss the decrease of the carrier concentration with increasing substrate temperature in regard to the formation of nonequilibrium defects.